The present invention relates to decimation of digital signal samples, and more particularly to decimation of digital signal samples in a telecommunications receiver.
Modern communications systems rely more and more on processing communicated signals by means of digital rather than analog technology. This trend is not confined to computer-based landline networks, but is increasingly finding its way into radio telecommunications systems because of the various efficiencies associated with digital technology. For example, so-called software radios use general-purpose processors or reconfigurable pieces of digital electronics to perform significant amounts of signal processing. This enables the form of radio protocol that governs receiving and transmitting to be substantially determined by the software that is run in the transceiver. In applications such as cellular telephone communication, this characteristic is quite useful because cellular telephones may be called upon to change radio protocols in real time.
Even in radios that utilizes significant amount of digital processing, the communicated signals are in analog form as they pass between the transmitter and the receiver by means of a channel. In a typical digital radio communication receiver, the received radio signal is first demodulated (down converted) to the baseband. While it would be desirable to perform this demodulation by means of digital processing, digital electronics are presently too slow to perform this function. The Nyquist sampling theorem dictates that an ideal software radio would have to collect and process samples at twice the maximum frequency at which it is to operate. Typical radio signals are often generated in the 1 to 2 GHz range. Present-day digital technology is simply not fast enough to perform at such a high rate, at least not with technology that would be practical for commercial distribution. Consequently, demodulation is typically performed by means of analog technology.
By contrast, the baseband analog signal, generated by demodulation, oscillates at a relatively low rate (e.g., at the baseband symbol rate). It is therefore quite feasible to sample this signal, and then convert the samples into the digital domain by means of an analog-to-digital (A/D) converter. In accordance with the Nyquist sampling theorem, the rate at which the sampling is performed will determine the highest frequency component that can be recovered from the digital signal samples. Frequency components higher than this highest frequency will cause a distortion in the digital signal, called “aliasing.” Because the analog signal being sampled often includes frequency components higher than the maximum recoverable frequency, the analog signal is typically first processed by an anti-aliasing filter whose purpose is to remove those excessively high frequency components.
The anti-aliasing filter is often unable to completely eliminate out-of-band noise and interference signals. To make up for this, the sampling rate selected for use in the analog-to-digital conversion process is very often higher than the rate required by the Nyquist sampling theorem. The use of a higher than necessary sampling rate is called “over-sampling.” The over-sampling rate is especially high when the A/D converter uses sigma-delta modulation, which uses the very high over-sampling rate to achieve higher resolution of the digital signal.
The use of a higher-than-necessary sampling rate results in more samples being generated than are actually necessary to recover the desired information imposed on the signal. Because the digital circuitry downstream of the A/D converter assumes the presence of a digital data stream generated at the Nyquist rate rather than the higher oversampling rate, the sampling rate of the digital signal generated by the A/D is reduced by systematically eliminating some of the samples, in a process called “decimation.”
FIG. 1 is a block diagram of a conventional arrangement for decimating a baseband signal. A low pass filter (LPF) 101 with a normalized cut-off frequency of π/M is used to reduce the bandwidth of the signal before the decimator 103 is applied, where M is the decimation rate. Signal samples of a pre-determined phase of the filtered poly-phase signal are preserved and the remaining samples are cast away. Very often, the decimation operation is integrated into the LPF to form a decimation filter having reduced operations.
The conventional decimation scheme tries merely to preserve the spectrum integrity of the desired baseband signal. To do this, the Nyquist principle is applied in a purely temporal perspective. However, in real world radio communication, a transmitted signal is often subjected to multipath propagation as it passes from the transmitter to the receiver. That is, the transmitted signal can fan out in many directions when it leaves the antenna. Some part of this signal may reach the receiver's antenna via a direct path. Other parts of the transmitted signal may not initially be directed to the receiver's antenna, but may eventually arrive there as a result of being reflected off of objects in the terrain. Because these reflected signals take a longer path to reach the receiver, they are somewhat delayed relative to a direct signal. The combination of all of these variously delayed signals at the receiver's antenna results in a kind of interference that can be corrected and beneficially used by the receiver.
Thus, the oversampled poly-phase signal at the output of the low pass filter 101 also contains spatial diversity information due to the delay spreading of the physical propagation channel. This diversity information is very important for extracting desired signal from a noisy environment and/or strong interference. By casting away all signal samples except those corresponding to the pre-determined phase, the spatial diversity information is lost in the conventional decimation scheme, which results in a lower Signal-to-Noise Ratio (SNR) for the decimated signal and causes degraded Bit-Error-Rate (BER) performance for the receiver.
It is therefore desirable to provide decimation processes and apparatuses that better make use of diversity information that is present in an oversampled poly-phase signal.